How do I combine these two Render Layers?
First, the word is 'node' :)

Second, Ensure you're rendering transparent layers for your leaf.


Third, the Viewer node is just a shortcut to see what the image looks like after a branch of other nodes have processed a node branch. To build both of these render layers into a finished picture you'll need a Composite node. The Viewer node just gives you the background under the node tree. You can use both: just feed both the Viewer and Composite nodes the same input.


Have fun :)